1. Field Of The Invention
This invention relates to piston-type internal combustion engines having a combustor/expander cylinder, a separate compressor, and an exhaust heat recuperator means which preheats the compressed air charge. The invention relates further to an engine having a recuperator and a recuperator-protective valve to protect the recuperator from the combustion process.
2. Description Of The Related Art
Internal combustion engines today, with the exception of Diesels, operate on what is commonly known as an Otto cycle originally patented in France in 1862 by Alphonse Beau de Rochas. In 1876, the Rochas compression cycle was incorporated into a practical engine by Nicholas A. Otto. Otto engines were immediately more efficient than Lenoir non-compressing gas engines which had been in production since 1862. Then in 1892, Rudolf Diesel invented the compression ignition engine with higher efficiency than an Otto engine. At the time, their efficiencies were about 3 to 4% for the Lenoir, 12% for the Otto, and 24% for the Diesel, and compared with their expansion ratios of approximately 1.5: 1, 2.5:1, and 16:1.
The low efficiencies are related to the large amount of energy remaining in the engine exhaust at the moment of release by the exhaust valve. Exhaust temperatures for example of 1,450.degree. Fahrenheit or more were reported for the Lenoir and Otto, and around 900.degree. F. for the Diesel. Actual gas temperatures inside the cylinders when expansion was complete were surely much higher. This is because a great deal of exhaust gas heat transfers to the exhaust valve and then to the exhaust port walls. For example, gas reaches about 90% equilibrium with wall temperature after flowing only ten diameters along the length of a straight pipe. In early engines, exhaust valves and exhaust ports were labyrinthine in design and thus much of the heat from the exhaust was absorbed by these parts before the exhaust exitted the engine.
The better efficiency of the Diesel came about due to its very high expansion ratio, a result of the high compression ratio needed to create high temperature (generally greater than 550.degree. F.) sufficient to auto-ignite the injected fuel. The high compression ratio and attendant gas and bearing pressures required greatly increased strength and with it, increased weight and cost. In fact, the Diesel is two to three times the weight and cost of a comparable Otto engine.
The first recuperative internal combustion engine of the prior art appears to be U.S. Pat. No. 4,155,087 issued on Sep. 15, 1874 to Joseph Hirsch. The described engine has two cylinders interconnected by a regenerator made of refractory elements. Because the heat exchanger is located in an external duct also made of refractory material, the heat exchanger continuously radiates away thermal energy. Hot exhaust gas from the "hot-air" cylinder, after passage through the heat exchanger, passes into the "cold-air" cylinder. When heated exhaust gas is in the cold-air cylinder, water is injected to cool and reduce the volume of the gas in the cold-air cylinder. Additional make-up air is then added under pressure and the gas volume is finally transferred to the hot-air cylinder via the heat exchanger. Utilization of the thermal energy in the exhaust is far from optimum as a consequence of lowering the temperature of the charge before transfer to the hot-air cylinder by way of the heat exchanger. Taking the radiation and convection heat losses from the heat exchanger into account, it is difficult to see how the device can effect an appreciable increase in the Carnot efficiency.
U.S. Pat. No. 328,970 issued Oct. 27, 1885 to James F. Place describes an engine having a compression-cylinder and a power-cylinder arranged in a vee, their cylinder heads connected by a regenerative means for the capture of exhaust heat. A cylindrical, internally finned, dual purpose recuperator valve of the type that is usually oil lubricated and sealed was located in the power cylinder head. This valve captured heat as it released exhaust gas and transferred the heat to the compressed charge passed back through the valve to the power-cylinder. Such a recuperator valve would probably have attained 1,000.degree. F., creating problems with its lubrication and with any associated seals. The second stage external tubular recuperator connecting the cylinder heads was of considerable length and presented a large area for loss of heat to the atmosphere instead of retention in the next cycle. Place's patent indicates an amazing understanding of the problem when one considers the date of his work.
U.S. Pat. No. 642,176 issued to E. Thomson describes a two cylinder engine with its cylinders interconnected by a recuperator which is not separated from either cylinder by valving. Air is inducted into one cylinder and the other cylinder is filled with a fuel-air mixture. The fuel-air mixture is inducted into the cylinder containing air via the recuperator and is ignited during passage through the recuperator. Exhaust is released from each cylinder directly to the atmosphere and not through the recuperator whereby no exhaust heat is recovered.
U.S. Pat. No. 870,720 issued to A. J. Frith describes a two cylinder engine with the cylinders coupled by a recuperator as in the device of Thomsom without the isolation from either cylinder by valving. This engine suffers from the same deficiency as the engine of Thomson, in that exhaust is transferred to a cylinder and released directly to atmosphere without passing through the recuperator as is needed for maximum recovery of thermal energy. Additionally, the inventor teaches that the air should be saturated with water prior to compression and passage through the recuperator to the other cylinder. The presence of water vapor will lower the temperature of the recuperator and further reduce the Carnot efficiency.
In U.S. Pat. No. 1,111,841 issued to J. Koenig and U.S. Pat. No. 1,904,070 issued to J. D. Morgan, both inventors have the similar idea of cooling compressed air followed by heating the air in a recuperator. This method of operation does not make full use of the high value thermal energy of the exhaust since cooled compressed air sent through the recuperator must result in a lower temperature of the output gas and must therefore reduce the maximum attainable working temperature in the engine. Thus, the Carnot efficiency of the engines of each of these two inventions is less than that which is desired and is believed to be attainable in a properly engineered internal combustion engine.
In United Kingdom patent 528,391 issued 10/1940 to Michael Martinka there is described an engine having a regenerative heat-exchanger mounted movably within the combustion chamber and thus exposed to the combustion gases.
In United Kingdom patent 640,410 issued 7/1950 to Isaac Lubbock and R. Rigby and assigned to Shell Refining and Marketing Co. Ltd. there is described an engine having a regenerator fixed in the combustion chamber, or alternatively fixed to the head of the piston, and in either case exposed to the combustion gases.
In United Kingdom patent 761,122 issued 11/1956 to Richard Rigby and assigned to Shell Refining and Marketing Co. Ltd., an engine is described having a regenerator in the cylinder and attached to a movable sleeve. Another engine is described having a regenerator fixed in a cylinder between two reciprocating pistons. In each case the regenerator is exposed to the combustion process which is generally above the temperature tolerance of known regenerator elements.
In Swiss patent 307,098 issued 5/1955 to J. H. Keller and assigned to N. V. Machinefabriek en Reparatiebedrijf describes an engine having a multistage, intercooled compressor followed by an external regenerator feeding into a combustion/expansion cylinder.
United Kingdom patent 1,308,355 issued 2/1973 to Daimler-Benz Aktiengesellschaft on Feb. 28, 1973 describes an engine having a regenerator located and exposed to combustion between dual opposed pistons. It states that by use of the best heat-resistant materials, the heat exchanger/regenerator may be operated at 1,200.degree. C.
United Kingdom patent 1,440,595 issued 6/1976 to W. C. Pfefferle and assigned to Engelhard Minerals & Chemicals Corporation describes two engines, one having a catalyst member located and exposed to combustion between dual opposed pistons, and a second having a catalyst member located in a combustion chamber above a single piston. U.S. Pat. No. 4,389,983 issued to B. E. Enga et al and assigned to Johnson, Mathey & Co., Ltd teaches a single piston engine having a catalytic unit located in a port connecting the cylinder with a precombustion chamber. In both inventions, temperature tolerance of the catalyst member or catalytic unit places an upper limit on the operating temperature within the combustion chamber.
U.S. Pat. No. 4,040,400 issued to Karl Keiner teaches that the low efficiencies of internal combustion engines are a consequence of the considerable heat losses associated with the expansion of the highly compressed gaseous media and with compression of the combustion supporting air in the cylinder. He proposes to increase efficiency by compressing the air charge in a cooled multistage reciprocating compressor which has recoolers between its discrete compression stages. He characterizes this type of cooled compression as being substantially isothermal. Once the air has been compressed it is heated by exhaust gases by passage through a coiled pipe in a chamber containing exhaust gas prior to mixture with fuel-air mixture during passage through a nozzle which imparts a whirl to the gases and causes self ignition. It is stated that super heated steam can be added to the compressed air or to the fuel-air mixture to prevent over heating in the combustion chamber. This combustion chamber being a part of the cylinder head that is open to, but thermally insulated from, the cylinder and above the top dead center of the reciprocating working piston. It is difficult to envision an increased Carnot efficiency when heat and shaft work is expended in the compression process and the maximum temperature of the combustion is deliberately lowered "to prevent overheating".
U.S. Pat. No. 4,074,533 issued to Thomas R. Stockton and assigned to the Ford Motor Company discloses three working cylinders interconnected by valved ducting to allow gas flow through all three cylinders in series. The middle cylinder in the series receives fuel above and displaced from a regenerator modified to act as a catalytic combustor. Greater efficiency would be realized if there were only one cylinder operating as a two stroke engine. Excessive pumping from one cylinder to another seems to be wasteful of useful work.
U.S. Pat. No. 4,133,172 issued to Roy S. Cataldo and assigned to General Motors Corporation teaches the use of interconnected piston-cylinder arrangements for compression and expansion of the working fluid with an exhaust heat recuperator and combustor positioned in series between the compression and expansion cylinder, wherein the gases discharged from the expansion cylinder flow to exhaust through the recuperator. It would appear from the two figures of the drawing that the air being heated by the recuperator passes through it at right angle to the direction of flow of the exhaust gases. Such arrangement would reduce the ability of the recuperator to effect maximum heating of the compressed air charge. In any event, the location of the recuperator and the combustor in an external uninsulated duct extending between the two cylinder heads of a Vee-engine presents a large surface for thermal convection and radiation losses to degrade the high temperature thermal energy available from the exhaust, while the occurrence of combustion adjacent the exposed recuperator would lead to deterioration of the element.
U.S. Pat. No. 4,630,447 issued to William T. Webber describes, as was also done by Thomson and by Frith, an engine having two cylinders coupled by a recuperator without separation from either cylinder by valving. Air inducted into the cold cylinder is compressed, passed through the recuperator into the hot side, mixed with fuel, combusted, expanded, and passed through the recuperator for further expansion in the cold cylinder, and then exhausted to atmosphere without passing through the recuperator. As with Thomson and Frith, Webber's engine suffers from potential degradation of the recuperator due to its direct exposure to combustion, as well as loss of thermal energy with the exhaust.
U.S. Pat. No. 4,715,326 issued to Robert H. Thring is the first of three patents to Thring in a span of nine years. The others are U.S. Pat. No. 5,050,570 and U.S. Pat. No. 5,499,605. All three patents are concerned with internal combustion engines that each contain a means for the recuperative recovery of heat from the exhaust for reuse in heating the working charge. In each of the patents the recuperator is situated in an exposed duct where radiation and convection losses will reduce the maximum temperature attainable with a consequent reduction in the Carnot efficiency.
U.S. Pat. No. 4,781,155 was issued to Helmut G. Brucker for an engine having a supercharger cylinder and a combustion cylinder connected by a duct wherein a regenerator is situated. This engine suffers from the same deficiency as the others where the regenerator is subject to heat loss because no provision has been made to minimize convection and radiation losses.
U.S. Pat. No. 5,085,179 issued to Henry B. Faulkner and assigned to Ingersoll-Rand Company teaches the uses of thermal recuperation in the same manner as U.S. Pat. No. 4,781,155 and suffers the same deficiencies.
U.S. Pat. No. 5,228,415 issued to Thomas E. Williams employs a shell and tube heat exchanger to extract heat from the exhaust and transfer it to the compressed air charge while the air is in transit from a compression cylinder to a combustion cylinder. The shell and tube heat exchanger acts as a recuperator but is less efficient and unless provided with heavy insulation, is not as efficient as a recuperator of the type having a hot end and a cold end.
One solution to the exhaust waste problem has been the turbo-expansive conversion of exhaust energy to rotative energy. Typically the rotative energy is used to drive a turbo-compressor for boosting input air pressure to the engine. Turbines however are not well suited to the pulsating exhaust flow from a single cylinder nor can they tolerate the aforementioned high temperature gas released at the exhaust valve of the Otto engine. For this reason, exhaust is generally collected from several cylinders through exposed pipes which moderate the pulsations and cool the gas to 1,400.degree. F. or lower before it enters the turbine. Such cooling wastes the bulk of the potential energy to the atmosphere and for this reason, exhaust turbines have not been particularly effective for raising the efficiency of Otto engines.
Another solution to the exhaust waste problem and one been put into extensive practice has been the turbo-expansive conversion of exhaust energy to rotative energy. Typically the rotative energy obtained from waste exhaust is used to drive a turbo-compressor for boosting input air pressure to the engine. Turbines are not well suited however to the pulsating exhaust flow from a single cylinder. They also cannot tolerate the high temperature gas released at the exhaust valve of the Otto engine. For this reason, exhaust is generally collected from several cylinders through exposed pipes which moderate the pulsations and cool the gas to 1,400.degree. F. or lower before it enters the turbine. Such cooling wastes the bulk of the potential energy to the atmosphere and for this reason, exhaust turbines have not been particularly effective for raising the efficiency of Otto engines.
A large amount of art was also found on a group of heat engines having recuperators for internal heat exchange, but with external combustion wherein combustion heat is passed through a working chamber wall to enter the working gas.
The problem with most of these cited internal combustion engines employing a heat exchanger or recuperator of one type or another is the large radiative and convective heat loss caused by the exposed location and the large surface area of the heat exchanging element. As pointed out in the discussion of individual patents, heat losses from the recuperator lower the Carnot efficiency. In the very few instances in the prior art where the recuperator is not subject to radiation and convection losses, the recuperator is located in the working cylinder or located in an internal duct that is directly connected with and open to the working cylinder. This means that the recuperator or equivalent is directly exposed to the flame front of the ignited charge, with a consequent shortening of its useful life.